On Wed, May 18, 2011 at 09:37:08AM +0530, Desai, Kashyap wrote:
> On Wed, 2011-05-04 at 17:23 +0530, Kashyap, Desai wrote:
> > The following code seems to be there in
> > /usr/src/linux/arch/x86/include/asm/io.h.
> > This is not going to work.
> >
> > static inline void writeq(__u64 val, volatile void __iomem *addr)
> > {
> > writel(val, addr);
> > writel(val >> 32, addr+4);
> > }
> >
> > So with this code turned on in the kernel, there is going to be race
> > condition
> > where multiple cpus can be writing to the request descriptor at the same
> > time.
> >
> > Meaning this could happen:
> > (A) CPU A doest 32bit write
> > (B) CPU B does 32 bit write
> > (C) CPU A does 32 bit write
> > (D) CPU B does 32 bit write
> >
> > We need the 64 bit completed in one access pci memory write, else spin lock
> > is required.
> > Since it's going to be difficult to know which writeq was implemented in
> > the kernel,
> > the driver is going to have to always acquire a spin lock each time we do
> > 64bit write.
> >
> > Cc: sta...@kernle.org
> > Signed-off-by: Kashyap Desai <kashyap.de...@lsi.com>
> > ---
> > diff --git a/drivers/scsi/mpt2sas/mpt2sas_base.c
> > b/drivers/scsi/mpt2sas/mpt2sas_base.c
> > index efa0255..5778334 100644
> > --- a/drivers/scsi/mpt2sas/mpt2sas_base.c
> > +++ b/drivers/scsi/mpt2sas/mpt2sas_base.c
> > @@ -1558,7 +1558,6 @@ mpt2sas_base_free_smid(struct MPT2SAS_ADAPTER *ioc,
> > u16 smid)
> > * care of 32 bit environment where its not quarenteed to send the entire
> > word
> > * in one transfer.
> > */
> > -#ifndef writeq
>
> Why not make this #ifndef CONFIG_64BIT? You know that all 64 bit
> systems have writeq implemented correctly; you suspect 32 bit systems
> don't.
>
> James
>
> James, This issue was observed on PPC64 system. So what you have suggested
> will not solve this issue.
> If we are sure that writeq() is atomic across all architecture, we can use it
> safely. As we have seen issue on ppc64, we are not confident to use
> "writeq" call.
So have you told the powerpc people that they have a broken writeq?
And why do you obfuscate your report by talking about i386 when it's
really about powerpc64?
--
Matthew Wilcox Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours. We can't possibly take such
a retrograde step."
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